Transmission of signals from the ground waves are restricted to 1,500 kHz. Why so?

Why the transmission of signals using sky waves restricted to frequencies greater than 30 MHz?

Arrange the following in the order of reception and transmission of sound wave from the ear drum: Cochlear nerve, external auditory canal, ear drum, stapes, incus, malleus cochlea.

A horizontal overhead power line is at a height of 4m from the ground and carries a current of 100 A from east to west. The magnetic field directly below it on the ground is (mu_(0)=4pixx10^(-7)TmA^(-1))

From the top of a tower, the angle of depression of a point on the ground is 60^@ . If the distance of this point from the tower is 1/(sqrt3+1) metres, then the height of the tower is :